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Probing Mysteries of the Universe: Calculating Spin of the Black Hole

Posted August 7, 2013

Black holes are one of the most mysterious creations in our Universe. This is why these objects fascinate lots of astrophysicists around the world. Some even say that understanding inner workings of a black hole could help answer many important questions about the space-time continuum, nature of gravity and other mysterious entities.

According to scientists, the most important parameters of black holes are mass and spin. And, even though the methods for measuring the mass of black holes have been developed some decades ago, the situation is not so simple when talking about another characteristic – black hole spin angular momentum.

X-rays emitted by swirling disks of matter hint at the speed at which a supermassive black hole spins. Credit:  JPL-CALTECH/NASA

X-rays emitted by swirling disks of matter hint at the speed at which a supermassive black hole spins. Credit: JPL-CALTECH/NASA

According to interesting article in Nature, supermassive black holes, which are typically residing in the centers of the galaxies, are the most problematic to deal with. On one hand, astronomers cannot get a clear view of them due to high density of objects between us and the center point of the galaxy. On the other hand, no direct observations of these objects are possible at all, since no observable light can escape from the so-called event horizons of the black holes. Instead scientists observe X-rays, which are emitted by matter orbiting around the black hole until eventually they transit the boundary of the event horizon.

Source: Christopher Reynolds, University of  Maryland

Source: Christopher Reynolds, University of Maryland

Only 19 supermassive black holes have had their mass and spin measured by astronomers until today which relatively sufficient degree of precision. However, in February of this year researchers from University of Cambridge, UK, reported their newest creation: novel method for supermassive black hole spin calculation. To prove its usability, they also calculated spin of one other supermassive black hole. Their work was featured in Nature.

At than time they had no possibility to verify the accuracy of this method. But authors said that their main aim was to introduce alternative method to measure important parameter of spinning black holes.

Current method used to measure spin was introduced in 1995 and is based on measurement of X-rays emitted by superheated accelerated matter spiraling around the event horizon. However, not all astronomers are positive regarding the accuracy of this method of spin measurement. The main cause of such shift in opinion is the fact that since the introduction of X-ray-based spin measurement technique, engineers have developed more sensitive X-ray measurement equipment which is used in modern space telescopes, like NASA’s NuSTAR. More precise detection of X-rays also means significant distortions in calculations used to determine the spin.

More recently, another team of astronomers from Durham University, UK, was able to obtain more precise spin measurement results, which were obtained by taking into account the lower-energy X-rays. The spectrum of these X-rays allows making substantiated conclusions about the temperature of matter located relatively close to the event horizon. And, since this temperature is an indirect estimate of black hole rotation velocity, researchers concluded that this particular black hole located approximately 150 million parsecs away spins at 86% of the speed of light.

Authors of the study think that their method is more realistic, since the ‘classic’ method used to determine the spin of black holes consistently gives the overestimated results, such as rotation speed above 90% of the speed of light. Certainly, not all astronomers tend to agree with such claim. Some of them are saying that the spin and mass of supermassive black holes may vary over the time.

And, there are some significant differences related to the magnitude of this physical characteristic. If we take the upper end of calculated spins, or black hole rotation velocities, this could mean that supermassive black holes could be formed as a result of colliding galaxies when significant amount of matter moving from one side increases the spin of that object. If we assume the lower range of spin calculation results, this could mean that galaxy-size collisions are not the prerequisite for the development of such phenomenon.

Story by Alius Noreika,

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